A Pilot-Aided, Time-Domain Doppler Estimator, Tracker and Compensator for Doubly Dispersive Underwater Acoustic Channels Dominated by Wave Motion
In this thesis, a time-domain Doppler estimator, tracker, and compensator using shift-orthogonal OFDM pilot sequences are designed based on the theory presented and tested using simulations and real-life experiments. OFDM signals are assumed and used throughout the dissertation, and a shift-orthogonal OFDM pilot sequence is designed. The UWA channel is analyzed and a simplified theoretical model is presented that holds under achievable conditions. The Doppler estimator is then developed, which uses an approach reminiscent of existing differential demodulation techniques. The estimator is developed with the flexibility of handling any Mach number provided the designer has the liberty of adjusting the signal's bandwidth, carrier frequency, or preamble size. The CRLB for the estimator is derived. The Doppler estimator performance is compared against existing estimators in literature and is shown to outperform most existing estimators in terms of MSE, with the added features of Doppler tracking and low computational complexity. The Mach number estimates also have a closed-form expression. A Doppler tracker is developed that reduces the size of the Mach number estimates' array while simultaneously tracking significant changes in the Mach number. A novel and practical Doppler compensator based on the proposed estimator is developed, which compensates the Doppler effect in two stages. The first stage involves resampling, which takes in the estimates at the tracker output and applies a form of time-varying resampling which we call block-by-block resampling. The second stage involves estimating the residual Doppler shift with the proposed Doppler estimator, followed by eliminating the residual shift via a simple phase rotation. The proposed tracker and compensator subsystem is shown to outperform most existing compensators in terms of MSE, while also being more computationally efficient. Each element of the proposed receiver subsystem is tested in simulations and the results are shown to agree with theory. Finally the full receiver subsystem is tested in real undersea experiments at various ranges, bandwidths and power levels, and the subsystem is shown to yield minimal residual errors when tracking and compensating the Mach number.